A. Field of the Invention
This invention is related to cutting tools for machining operations and, more particularly, a method and composition for laminated ceramic cutting tools.
B. Description of the Prior Art
Metal cutting or "machining" is a widely used process in manufacturing. Among the common machining operations are shaping, planing, milling, facing, broaching, grinding, sawing, turning, boring, drilling, and reaming. Some of these processes, such as sawing, operate on both the external and internal surfaces of the workpiece, while others operate only on the internal (for example, reaming) or external (for example, milling) surfaces of the workpiece.
A wide variety of materials have been used or suggested as cutting tools, such as tool steels, high-speed steels, cast non-ferrous alloys, sintered carbides, diamonds and ceramics. The commonly measured parameters of cutting tool performance are cutting speed, depth of cut, feed rate, and tool life. Each of the prior art cutting tool materials is deficient in one or more of these parameters. Cutting tools made from tool steel, high-speed steel and cast non-ferrous alloys all have critical temperature limitations which restrict their cutting speed to relatively low rates, as measured in feet per minute (fpm). Typically, high-speed steels are restricted to 100-225 fpm for cutting steel and 250-300 fpm for cutting non-ferrous materials. The cast non-ferrous alloys will operate at up to about twice those rates. The carbide materials, such as tungsten carbide, improve on the cutting speed rates of the steels by a factor of 2-5, particularly when the carbides are coated. However, the carbides are not as tough as the steels and are susceptible to impact breakage. This severely limits their usage in applications where impact is a factor, such as in making interrupted cuts or in machining hard workpieces.
Ceramic cutting tools were first used around 1905 and have been gaining increasing acceptance in the United States since the 1950's. Ceramic materials, such as aluminum oxide (Al.sub.2 O.sub.3) , also known as "alumina", have been found to produce cutting tools which can operate at much higher speeds than the conventional steel and carbide cutting tools. There has been a trend to substitute ceramic-based cutting tools for metal cutting tools in the machine tool industry. Ceramic and ceramic composite compositions most often used for cutting tool applications include alumina, silicon nitride (Si.sub.3 N.sub.4), silicon nitride based ceramics in which aluminum is partially substituted for silicon and oxygen is partially substituted for nitrogen (the so-called "sialons") and aluminum oxide composite with titanium carbide (Al.sub.2 O.sub.3 /TiC).
Ceramics possess many but not all of the desirable properties required for cutting tools and have some of the characteristics necessary to counteract the principal causes of wear on cutting tools. The principal advantages of ceramic cutting tools are hardness, stiffness, high temperature strength, and chemical stability at elevated temperatures. Chemical stability is particularly important in minimizing the creation of craters on the top surface of the cutting tool, known as the rake face. High temperature strength is desirable during extended cutting operations. The main limitations of ceramics for cutting tools is their low tensile strength, low fracture toughness, and low impact resistance and thermal shock resistance. These property limitations make ceramic cutting tools of the prior art prone to premature failure by chipping, cracking or edge failure.
Reinforced ceramic cutting tools have recently been developed to improve on the cutting speed and life of the ceramic cutting tool. U.S. Pat. No. 4,961,757 to Rhodes et al. describes a reinforced ceramic cutting tool wherein the tool is comprised of a ceramic matrix reinforced by ceramic whiskers. The ceramic matrix is alumina or silicon nitride, and the whiskers are preferably silicon carbide. In tools according to the Rhodes et al. patent, silicon carbide whiskers are directly blended with the ceramic matrix and the mixture is hot pressed into cutting tools. A problem with this method and composition for ceramic cutting tools is that it provides very little control over the distribution and orientation of the reinforcements in the final tool body. The reinforcements are present throughout the entire thickness of the tool, and they have a substantially random orientation in the plane normal to the pressing direction and a partially random orientation in the through-thickness direction.
Another significant limitation to the method and composition of the reinforced ceramic cutting tools set forth in Rhodes et al. is the cost of reinforcements such as silicon carbide whiskers. Whiskers cost ten times that of the common matrix materials (for example, aluminum oxide). Hence, cost-effective manufacturing methods and material designs that make efficient use of the reinforcements, their distribution and orientation, are required. Cutting tools purportedly pursuant to the Rhodes et al. patent are commercially available under the designation "WG-300", as marketed by Greenleaf Corp., Saegertown, Pa.
The inventor has published articles generally discussing the properties of laminated ceramic composites and the reinforcement of these composites. See Amateau, "Properties of Laminated Ceramic Composites", record of proceedings for the 37th Sagamore Army Materials Research Conference, held Oct. 1-4, 1990, pp. 327-338; Kragness, Amateau and Messing, "Processing and Characterization of Laminated SiC Whisker Reinforced Al.sub.2 O.sub.3 ", Journal of Composite Materials, Vol. 25, p. 416 (April 1991); Kim, Amateau and Messing, "Residual Stresses in SiC Whisker-Mullite Laminated Composites", Ceramic Transactions, Vol. 19, p. 677 (1991); and Wu, Messing and Amateau, "Laminate Processing and Properties of Oriented SiC Whisker-Reinforced Composites", Ceramic Transactions, Vol. 19, p. 665 (1991). However, none of these publications teaches or suggests the use of reinforced laminated ceramic composites for cutting tool applications.
As recognized at column 3, lines 35-40 of the Rhodes et al. patent, the critical and unique combination of operating stress, temperature and impact makes the determination of materials suitable for use as cutting tools significantly different from the determination of materials for other industrial articles, such as heat exchangers, gears, refractories, heat engines and armor.
Finally, in Amateau and Messing, "Laminates, Ceramic", International Encyclopedia of Composites, Vol. 3, VCH, New York, pp. 11-16 (1990), the inventor pointed out that ceramic composites have been used in cutting tool bits and stated that laminated composite designs would permit economical use of the high-cost reinforcement phases as well as improving fracture strength and impact resistance. However, this article made no mention of any particular designs or processes for making laminated ceramic cutting tools. The inventor is aware of no specific teaching or suggestion in the literature of the art, or in commercial applications, for using a laminated, reinforced ceramic, wherein the thickness, composition and arrangement of the lamina and the orientation of the reinforcements is controlled to maximize the use of the expensive reinforcement materials and to improve properties which are important to cutting tools. The marked improvement in cutting tool performance realized when using properly reinforced laminated ceramic cutting tools, in light of the dearth of information in the literature or in practical applications in this regard, indicates that it would not have been obvious to one of ordinary skill in the art to utilize the method and device for laminated ceramic tools as hereinbelow described and claimed.
Therefore, it is an object of the present invention to provide a reinforced laminated ceramic cutting tool and a method of manufacturing the same wherein the distribution and orientation of reinforcements in the tool body is controlled so that the cost of the reinforcements is minimized while the enhancement of the properties of the cutting tool is maximized. It is a further object to provide a laminated ceramic cutting tool wherein the thickness, composition and arrangement of the lamina are selected to improve properties which are important to cutting tools. It is a still further object to increase tensile strength, fracture toughness, impact resistance and thermal shock resistance, as well as resistance to chemical wear, for ceramic cutting tools. Finally, it is an object to provide a method and device for laminated ceramic cutting tools wherein the tool exhibits greatly reduced wear per unit time when compared to prior art reinforced ceramic composite cutting tools.